1. Field of the Invention
This invention relates to a styrene-based polymer composition, a stretched molding thereof and a process for producing the same. More specifically, it relates to a styrene-based polymer composition which is suitable for materials such as the exterior trim parts of an automobile, engine compartment parts, machine parts, electric and electronic parts, domestic kitchenware, etc. a stretched molding thereof and a process for efficiently producing the stretched molding.
2. Description of the Related Arts
Heretofore, a composition which comprises a polyphenylene ether resin (polyphenylene oxide) (hereinafter referred to as "PPE") and a styrene-based polymer has been widely used as engineering plastics which are excellent in mechanical properties and heat resistance. Such compositions have been described in, for example, Japanese Patent Applications Laid-Open No. 5085/1971 and No. 39457/1972, and U.S. Pat. No. 3,383,435.
However, these compositions have serious drawbacks in that solvent resistance, particularly resistance to aromatic hydrocarbon type solvents, is small, whereby their uses are limited. This can be considered as an essential drawback belonging to the styrene-based polymer having atactic configuration as one component of the composition.
Also, PPE generally has self-extinguishing properties and therefore, a resin composition containing PPE also has flame retardance to some degree. However, demands on flame retardance in electric and electronic fields and automobile fields in recent years has become increasingly severe, so that the above demands cannot be satisfied with the flame retardance due to the self-extinguishing properties which PPE originally has.
In general, styrene-based polymers have been prepared by radical polymerization. The styrene-based polymers thus prepared have atactic configuration in streoregularity. However, in such styrene-based polymers, improvement in physical properties by stretching cannot be expected. Also, styrene-based polymers having isotactic configuration in stereoregularity have also been known and an attempt to stretch the polymer has been made (Polymer Chemistry, vol., 21, p. 206 (1964)). However, the styrene-based polymers have a slow crystallization rate and its crystalline structure is helical so that a sufficient stretching effect cannot be essentially obtained.
The group of the present inventors' has proposed a process for producing a film-like molding by stretching a styrene-based polymer having mainly syndiotactic configuration and a resin composition comprising the above styrene-based polymer and other thermoplastic resins and/or rubber in Japanese Patent Applications No. 3847/1988, No. 4921/1988, No. 4923/1988 and No. 4924/1988. In these applications, a low molecular weight PPE having a molecular weight of 10,000 or less is disclosed as one example of the thermoplastic resin.
However, after further investigation concerning the composition, it has found that there is room for further improvement since mechanical properties, solvent resistance and flame retardance are insufficient due to the low molecular weight of PPE.
Also, as proposed in the above Japanese Patent Application No. 3847/1988, the above styrene-based polymer becomes a stretched molding having excellent transparency and high modulus of elasticity by stretching. However, this stretched molding has a disadvantage in that its mechanical strength is low, as compared with stretched polyester film or polyamide film. Therefore, in order to improve its mechanical strength, an attempt of blending the aforesaid styrene-based polymer having syndiotactic configuration with other resins (particularly a low molecular weight PPE) has been made (Japanese Patent Application No. 4921/1988).
However, according to research thereafter, it has been found that this blended material is still insufficient in mechanical strength of its stretched molding due to the low molecular weight of PPE.